This invention relates generally to power conversion and, more particularly, to a series resonant converter which includes an L-C circuit in parallel with a tank circuit of the converter to prevent current from flowing in body diodes of the power transistors.
In recent years, resonant converters have attracted increased attention due, in part, to their high efficiency, low switching losses, low levels of electromagnetic interference (EMI), the ability to optimize the design of filters and magnetic components at a specific frequency, and their conduciveness toward miniaturization.
One fixed-frequency regulation method is pulse width modulation (PWM) of the resonant tank voltage; this method is also known as phase shift pulse width modulation (PWM), and a converter using this method may also be called a clamped-mode resonant converter. One such prior art resonant converter with PWM is shown in FIG. 1. Switches S1 and S2 are switched alternately with a fifty percent duty cycle, and switches S3 and S4 are similarly switched. In order to achieve PWM control of the resonant tank voltage, turn-ON of S4 is delayed with respect to the turn-ON of S1 by a phase angle which is dependent on the current requirements of the load. Turn-ON of S3 is similarly delayed with respect to the turn-ON of S2.
Switching type regulating devices used in power converters utilize semiconductor devices for the switching devices, such as metal-oxide semiconductor field effect transistors (MOSFETs). These devices are turned-ON or saturated of turned-OFF during operation. When the MOSFETs are turned-ON fully, the semiconductor devices are conducting and little or no power is dissipated. Also, when nonconducting or fully OFF, no power is dissipated. However, power is dissipated in such a semiconductor device during the time interval of switching from a nonconducting condition to a conducting condition and vice versa. It is during the transition or switching time interval that a substantial amount of power may be dissipated in such a semiconductor device, and if large enough such power may severely damage the semiconductor device.
In order to maintain low switching losses, this circuit must be operated in a discontinuous conduction mode. However, with a converter design that requires high voltage outputs, continuous conduction is preferred. The antiparallel diodes shunting the switches are forced to turn-OFF while the current still flows, and because of their relatively long recovery times, considerable turn-ON losses are produced in the switches.
This condition of turn-ON losses can result in a failure if power MOSFETs are used as switches and their integral body diodes serve as the antiparallel diodes. If for example, S1 turns-ON when the antiparallel diode shunting S2 still conducts, a "shoot-thru" current occurs and a potentially destructive dv/dt can develop across S2. If the failure does not occur, the turn-ON losses or extra power dissipation become excessive due to long recovery time of the MOSFET's integral body diodes and prevent the converter from operating at higher frequencies. If reduced physical size of a converter is important, then operating at a higher frequency becomes a requirement.
Even with high speed blocking and antiparallel diodes connected to all switches in order to eliminate the "shoot-thru" condition, large voltages and currents can be developed by the resonant tank during an overload. A fast current foldback or shutdown protection is required in order to prevent catastrophic failure of the converter under the short circuit condition.